Radiograms have been produced by using layers of radiation sensitive materials to directly capture radiographic images as image-wise modulated patterns of electrical charges. Depending on the intensity of the incident x-ray radiation, electrical charges generated either electrically or optically by the x-ray radiation within a pixelized area are quantized using a regularly arranged array of discrete solid-state radiation sensors.
U.S. Pat. No. 5,319,206 describes a system employing a layer of photoconductive material to create an image-wise modulated areal distribution of electron-hole pairs which are subsequently converted to corresponding analog pixel (picture element) values by electro-sensitive devices, such as thin-film transistors. U.S. Pat. No. 5,262,649 describes a system employing a layer of phosphor or scintillation material to create an image-wise modulated distribution of photons which are subsequently converted to a corresponding image-wise modulated distribution of electrical charges by photosensitive devices, such as amorphous silicon photodiodes. These solid-state systems have the advantage of being useful for repeated exposures to x-ray radiation without consumption and chemical processing of silver halide films.
In systems utilizing a photoconductive material such as selenium such as the prior art, a conventional radiation imaging system 100 shown in FIG. 1, before exposure to image-wise modulated x-ray radiation, an electrical potential is applied to the top electrode 110 to provide an appropriate electric field. During exposure to x-ray radiation, electron-hole pairs are generated in the photoconductive layer 190, under the dielectric layer 120, in response to the intensity of the image-wise modulated pattern of x-ray radiation, and these electron-hole pairs are separated by the applied biasing electric field supplied by a high voltage power supply. The electron-hole pairs move in opposite directions along the electric field lines toward opposing surfaces of the photoconductive layer 190. After the x-ray radiation exposure, a charge image is received at the charge-collection electrode 130 and stored in the storage capacitor 160 of the transistor 150, which is formed on the substrate 170. This image charge is then readout by an orthogonal array of thin film transistors and the charge integrating amplifier 140. This type of direct conversion system has the distinct advantage of maintaining high spatial resolution more or less independent with the thickness of the x-ray converting photoconductive layer. However, currently, only a very limited number of direct converting photoconductors can be used for commercial products.
The most popular and technical matured material is amorphous selenium that has good charge transport properties for both holes and electrons generated by the x-ray. However, selenium having an atomic number of 34 has only good x-ray absorption in the low energy range, typically below 50 KeV. The absorption coefficient of selenium at higher energy x-ray is smaller and therefore thicker selenium layer is required for adequate x-ray capture. Since the complication and difficulty of fabrication of good imaging quality amorphous selenium is a strong function of the selenium thickness, successful x-ray imaging products so far are limited to lower energy x-ray application such as mammography, low energy x-ray crystallography, and low energy non-destructive testing.
For high energy or high intensity x-ray applications, a large number of electron hole-pairs can be generated from each absorbed x-ray photon. When the electrons and holes move along the electric field to the charge collecting electrodes or to the bias electrode, a significant number of electrons and/or holes can be trapped in the selenium layer. These trapped charges will alter the local electric field, and therefore the subsequent charge transport and charge generation efficiency, resulting in a shadow of the previous image superimposed on the subsequent image in a phenomenon known as “ghosting”. Certain image erasing processes are in general required to remove these charges and to restore the selenium layer to uniform charge conversion properties.
After exposure to a first x-ray, selenium experiences charge trapping, and therefore, it suffers from the ghosting effect. Due to these unwanted results, an erase process is needed to reduce the ghosting. K-band radiation from amorphous selenium can also deteriorate image resolution.
It is therefore desirable to design a radiation imaging system without loss of resolution, and with minimized ghosting in high x-ray radiation energy or high dose.